Apparatus for measuring microwave electromagnetic fields

Abstract

A microwave signal is transmitted from a transmitting antenna through a transfer medium to an electrically modulated and mechanically spun microwave scatterer. The scatterer may include an electric field responsive antenna, such as a dipole, for measurement of the electric field, or a magnetic field responsive antenna, such as a loop, for measurement of the magnetic field. An impedance within the scatterer is electrically modulated at an audio frequency, and the scatterer is mechanically spun at an angular frequency substantially below that at which it is electrically modulated. The scatterer thereby re-radiates to a receiving antenna a signal at the mocrowave source frequency which is modulated at both the frequency of electrical modulation and the mechanical spinning frequency of the scatterer. Signals from the microwave source and from the receiving antenna are combined to yield an output signal having a magnitude which is a function of the magnitude of the field received by the scatterer and which is phase shifted proportional to the phase shift of the microwave signal from the transmitting antenna to the scatterer and thence to the receiving antenna. The output signal may be compared with a signal coherent with the frequency of rotation of the scatterer to determine the tilt angle of the electric or magnetic field received by the scatterer.

I claim: 
     
       1. Apparatus for measuring the microwave field radiated from an antenna, comprising: (a) a microwave source;   (b) a transmitting antenna for radiating microwaves from said source and for receiving microwave radiation;   (c) a scattering electric field probe positionable in the radiated field of said transmitting antenna to receive microwave radiation therefrom and re-radiate a microwave signal back to said antenna, said field probe comprising: (1) a scattering antenna selectively responsive to the electric field component of the radiated microwave field,   (2) a modulatable impedance electrically connected in said scattering antenna,   (3) means for modulating said impedance at a frequency substantially less than the frequency of said microwave source,   (4) means for spinning said scattering antenna about an axis of rotation at an angular frequency substantially less than the frequency of modulation of said impedance;     (d) phase insensitive means for receiving a signal from said microwave source and the microwave signal from said scattering probe received by said transmitting antenna, and for combining and demodulating the signals to yield an output signal at the frequency at which said scattering antenna impedance is modulated and phase shifted proportional to the phase shift of the microwave signal received by said transmitting antenna, and with a magnitude which is independent of such phase shift and a function of the magnitude of the electric field received by said scattering antenna; and   (e) means for comparing a signal at a frequency coherent with the frequency of spinning of said scattering antenna with the output signal from said phase insensitive means, to thereby allow measurement of the relative tilt angle of the major axis of the microwave electric field received by said scattering antenna.   
     
     
       2. Apparatus for measuring the microwave field radiated from an antenna, comprising: (a) a microwave source;   (b) a transmitting antenna for radiating microwaves from said source;   (c) a receiving antenna for receiving microwave radiation;   (d) a scattering electric field probe positionable in the radiated field of said transmitting antenna to receive microwave radiation therefrom and re-radiate a microwave signal to said receiving antenna, said field probe comprising: (1) a scattering antenna selectively responsive to the electric field component of the radiated microwave field,   (2) a modulatable impedance electrically connected in said scattering antenna,   (3) means for modulating said impedance at a frequency substantially less than the frequency of said microwave source,   (4) means for spinning said scattering antenna about an axis of rotation at an angular frequency substantially less than the frequency of modulation of said impedance;     (e) phase insensitive means for receiving a signal from said microwave source and the microwave signal from said scattering probe received by said receiving antenna, and for combining and demodulating the signals to yield an output signal at the frequency at which said scattering antenna impedance is modulated and phase shifted proportional to the phase shift of the microwave signal received by said receiving antenna, and with a magnitude which is independent of such phase shift and a function of the magnitude of the electric field received by said scattering antenna; and   (f) means for comparing a signal at a frequency coherent with the frequency of spinning of said scattering antenna with the output signal from said phase insensitive means, to thereby allow measurement of the relative tilt angle of the major axis of the microwave electric field received by said scattering antenna.   
     
     
       3. Apparatus for measuring the microwave field radiated from an antenna, comprising: (a) a microwave source;   (b) a transmitting antenna for radiating microwaves from said source and for receiving microwave radiation;   (c) a scattering magnetic field probe positionable in the radiated field of said transmitting antenna to receive microwave radiation therefrom and re-radiate a microwave signal back to said antenna, said field probe comprising: (1) a scattering antenna selectively responsive to the magnetic field component of the radiated microwave field,   (2) a modulatable impedance electrically connected in said scattering antenna,   (3) means for modulating said impedance at a frequency substantially less than the frequency of said microwave source,   (4) means for spinning said scattering antenna about an axis of rotation at an angular frequency substantially less than the frequency of modulation of said impedance;     (d) phase insensitive means for receiving a signal from said microwave source and the microwave signal from said scattering probe received by said transmitting antenna, and for combining and demodulating the signals to yield an output signal at the frequency at which said scattering antenna impedance is modulated and phase shifted proportional to the phase shift of the microwave signal received by said transmitting antenna, and with a magnitude which is independent of such phase shift and a function of the magnitude of the magnetic field received by said scattering antenna; and   (e) means for comparing a signal at a frequency coherent with the frequency of spinning of said scattering antenna with the output signal from said phase insensitive means, to thereby allow measurement of the relative tilt angle of the major axis of the microwave magnetic field received by said scattering antenna.   
     
     
       4. Apparatus for measuring the microwave field radiated from an antenna, comprising: (a) a microwave source;   (b) a transmitting antenna for radiating microwaves from said source;   (c) a receiving antenna for receiving microwave radiation;   (d) a scattering magnetic field probe positionable in the radiated field of said transmitting antenna to receive microwave radiation therefrom and re-radiate a microwave signal to said receiving antenna, said field probe comprising: (1) a scattering antenna selectively responsive to the magnetic field component of the radiated microwave field,   (2) a modulatable impedance electrically connected in said scattering antenna,   (3) means for modulating said impedance at a frequency substantially less than the frequency of said microwave source,   (4) means for spinning said scattering antenna about an axis of rotation at an angular frequency substantially less than the frequency of modulation of said impedance;     (e) phase insensitive means for receiving a signal from said microwave source and the microwave signal from said scattering probe received by said receiving antenna, and for combining and demodulating the signals to yield an output signal at the frequency at which said scattering antenna impedance is modulated and phase shifted proportional to the phase shift of the microwave signal received by said receiving antenna, and with a magnitude which is independent of such phase shift and a function of the magnitude of the magnetic field received by said scattering antenna; and   (f) means for comparing a signal at a frequency coherent with the frequency of spinning of said scattering antenna with the output signal from said phase insensitive means, to thereby allow measurement of the relative tilt angle of the major axis of the microwave magnetic field received by said scattering antenna.   
     
     
       5. The apparatus of claim 1 or 3 including means for comparing the phase of the output signal from said phase insensitive means with a signal coherent with the frequency at which said scattering antenna impedance is modulated, to thereby allow measurement of the phase shift of the microwave signal from said transmitting antenna to said scattering antenna and back again. 
     
     
       6. The apparatus of claim 2 or 4 including means for comparing the phase of the output signal from said phase insensitive means with a signal coherent with the frequency at which said scattering antenna is modulated, to thereby allow measurement of the phase shift of the microwave signal from said transmitting antenna to said scattering antenna and thence to said receiving antenna. 
     
     
       7. The apparatus of claim 1, 2, 3 or 4 including an envelope detector receiving the output signal from said phase insensitive means and providing a signal proportional to the magnitude of the microwave field component received by said scattering antenna. 
     
     
       8. The apparatus of claim 1 or 2 wherein said scattering antenna comprises an electric dipole. 
     
     
       9. The apparatus of claim 3 or 4 wherein said scattering antenna comprises a loop of electrical conductor. 
     
     
       10. The apparatus of claim 1, 2, 3 or 4 wherein said means for comparing a signal to measure the tilt angle of the major axis compares the envelope of the magnitude of the output signal from said phase insensitive means with the signal at a frequency coherent with the frequency of spinning. 
     
     
       11. The apparatus of claim 5 wherein said means for comparing a signal to measure the tilt angle of the major axis compares the phase shift of the microwave signal as measured by said means for comparing the phase of the output signal with the signal at a frequency coherent with the frequency of spinning. 
     
     
       12. The apparatus of claim 6 wherein said means for comparing a signal to measure the tilt angle of the major axis compares the phase shift of the microwave signal as measured by said means for comparing the phase of the output signal with the signal at a frequency coherent with the frequency of spinning. 
     
     
       13. The apparatus of claim 1 or 3 wherein said phase insensitive means includes first and second balanced mixers, said first balanced mixer receiving a signal from said microwave source and the microwave signal from said scattering probe received by said transmitting antenna, the second of said balanced mixers receiving a signal from said microwave source phase shifted by 90° and the microwave signal from said scattering probe received by said transmitting antenna, said phase insensitive means further including a phase shifter which phase shifts by 90° the output of said second balanced mixer and an adder which adds the output signals from said phase shifter and from said first balanced mixer, and further including a narrowband amplifier centered at the impedance modulation frequency which receives and amplifies the output signal from said adder. 
     
     
       14. The apparatus of claim 2 or 4 wherein said phase insensitive means includes first and second balanced mixers, said first balanced mixer receiving a signal from said microwave source and the microwave signal from said scattering probe received by said receiving antenna, the second of said balanced mixers receiving a signal from said microwave source phase shifted by 90° and the microwave signal from said scattering probe received by said receiving antenna, said phase insensitive means further including a phase shifter which phase shifts by 90° the output of said second balanced mixer and an adder which adds the output signals from said phase shifter and from said first balanced mixer, and further including a narrowband amplifier centered at the impedance modulation frequency which receives and amplifies the output signal from said adder. 
     
     
       15. The apparatus of claim 1, 2, 3 or 4 wherein said means for spinning said scattering antenna includes a synchronous electric motor having its output shaft connected to the axis of rotation of said scattering antenna. 
     
     
       16. The apparatus of claim 1, 2, 3 or 4 wherein said modulatable impedance is a diode. 
     
     
       17. The apparatus of claim 1, 2, 3 or 4 wherein said modulatable impedance has a negative resistance region. 
     
     
       18. The apparatus of claim 8 wherein the size of said dipole is selected such that said dipole is self resonant at a chosen microwave frequency. 
     
     
       19. The apparatus of claim 9 wherein the size of said loop is selected such that said loop is self resonant at a chosen microwave frequency. 
     
     
       20. The apparatus of claim 1, 2, 3 or 4 wherein said modulatable impedance includes a photocell, and wherein said means for modulating includes a source of high frequency chopped light directed to impinge on said photocell. 
     
     
       21. The apparatus of claim 1, 2, 3 or 4 wherein said means for modulating includes an oscillator mounted to spin with said scattering antenna and electrically connected to said modulatable impedance, a radio frequency transmitter mounted to spin with said scattering antenna and connected to broadcast the impedance modulation signal, and a receiver for receiving and detecting the impedance modulation signal broadcast by said transmitter.